According to a related art electro-photographic image forming apparatus, when an image formed on a photosensitive member (image carrying unit) is transferred to a sheet, adjustment is made such that a suitable transfer current flows between the photosensitive member and a transfer unit, for example, a transfer roller. In order to appropriately adjust the transfer current, it is necessary to accurately calculate load resistance of an application unit that applies a transfer voltage to the transfer roller.
The above described related art image forming apparatus discloses a technique to measure load resistance accurately. For example, according to the above technique, when a predetermined voltage is applied and a current is measured immediately after a recording medium enters the transfer roller, a leak current is measured at a portion, such as a sheet feed roller. And then, a current actually flowing in the transfer roller is calculated on the basis of the measurement result, thereby measuring accurate load resistance.